1. Field of the Invention
This invention relates to the providing of organometallic vapor for the manufacture of semiconductors.
2. Prior Art
For MOCVD growth, organometallic compounds are the source materials for the compound semiconductor industry. The organometallic compounds that are typically used as precursors for the chemical vapor deposition include trimethylaluminum (TMAI), trimethylgallium (TMG), triethylgallium (TEG), trimethylantimony (TMSb), dimethyl hydrazine (DMHy), trimethylindium (TMI), and cyclopetadienylmagnesium (Cp2Mg).
Typically, a volatile organometallic compound for vapor phase deposition is provided in a bubbler and subjected to a constant temperature wherein a carrier gas, such as hydrogen or nitrogen, is introduced to transport the compound and deliver it to a vapor phase deposition chamber.
A good organometallic precursor delivery technique provides a gas stream with a known, constant, controllable amount of contained organometallic. For liquid organometallic compounds, this is generally a straightforward exercise since mass transport and vaporization kinetics are fast enough to provide near-saturation concentrations at most reasonable flow rates of carrier gas. The term “bubbler” is used generically in the CVD industry to refer to any container for a precursor utilized for delivery of a carrier gas saturated in the aforementioned precursor.
In the case of solid organometallics, notably TMI, obtaining consistent delivery has proved to be an ongoing problem. In this case, vaporization kinetics are slower and dependent on issues such as product morphology, temperature, gas contact time, and surface area. Finely divided, irregularly shaped material with a high surface area will evaporate faster than a uniform, dense, more massive material. Mass transport is also more problematic. It is important to allow sufficient contact time for the slower evaporation process, and it is important to keep the carrier gas moving across all exposed surfaces at rates sufficient to deliver the appropriate amount of precursor to the CVD chamber. Channeling, for instance, will reduce both contact time and the area exposed to the flowing gas. Other factors, such as pressure changes as the carrier gas flows through the precursor bed, are known to cause erratic delivery rates and changes in degree of carrier gas saturation.
It is also highly desirable to provide a constant and stable delivery of vapor from a solid organometallic precursor at a near-saturation concentration in compound semiconductor device manufacture. Unstable vapor delivery rates for solid organometallic precursors are affected by a number of factors:                Reduction of total surface area of the solid is in progress continuously due to the depletion of the solid precursor. Small, high surface area particles are evaporated preferentially, causing rapid reduction of surface area early in the lifetime of the bed.        Channeling which may occur due to the erosion of the solid precursor bed.        Changes in pressure inside the bed during operation.        Grain growth effects due to the agglomeration process which occur with the evaporation and sublimation of the solid material along with a concurrent equilibrium in which redeposition occurs on solid precursor surfaces. At gas saturation, evaporation and redeposition occur at the same rate but bed morphology changes to favor lower surface area.        Gas paths become shorter and the available surface area becomes reduced as the solid precursor is consumed. Therefore, saturation of the carrier gas with the vapor of organometallic precursor becomes increasingly unlikely.        
An ideal bubbler design has to overcome the aforementioned problems and needs to achieve the following goals:                Provide a stable, constant vapor delivery rate until substantially complete depletion of the solid organometallic in the bubbler takes place.        Provide saturation or near-saturation concentration at most common and reasonable operating parameters such as temperature, pressure, carrier gas type (N2, H2, etc.) and flow rate of the carrier gas.        Provide fast response and fast re-establishment of a stable, constant vapor delivery rate when operating parameters are changed.        
There are known general approaches to the delivery of vapors from solid organometallics:    1) Solution TMI: The drawbacks noted in the industry when using “solution TMI” include entrainment of aerosols of the solvent and inconsistent and changing delivery rates for total Indium when using TMI/TEI.            a) U.S. Pat. No. 5,232,869 (1993): as practiced by Epichem. In this case, a suspended liquid is used to overcome kinetic and mass transport. Solid precursor dissolves into the solvent as it is depleted by evaporation to maintain equilibrium conditions and consistent delivery rates.        b) U.S. Pat. No. 5,502,227 (1996): TMI dissolved in R3In, such as triethylindium (TEI)            2) Another general approach is a bubbler design that improves the uniformity of flow and solid-gas contact in the bubbler. Strategies that have hitherto been employed include:            a) U.S. Pat. No. 4,916,828 (1990): The use of TMI mixed or dispersed with a “packing”.        b) U.S. Pat. No. 4,734,999 (1987): The use of a bubbler incorporating a dip tube fitted with a frit distributor at the end of the tube and with a reduced bubbler diameter at the bottom versus at the top.        c) U.S. Pat. No. 5,019,423 (1991): This design uses a carrier gas flowing upwardly through a packed bed of solid organometallic on top of a partition containing a plurality of pores.        d) U.S. Pat. No. 4,947,790 (1990): A carrier gas flows in the direction of the gravitation force in the following sequence: through a thick gas inlet plate, a powder solid bed, and a thin gas outlet plate.        e) PCT Patent Publication No. WO 99/28532 (1999): ultrasonic vaporizer is used.        f) U.S. Pat. No. 5,603,169 (1997): The use of a bubbler containing an exhaust tube, a compressing plate and a pair of porous thin plates is described.        g) U.S. Published Patent Application No. 2002/0078894 A1 (2002): this bubbler contains a metal sintered filter rather than a conventional dip tube.        h) U.S. Pat. No. 5,553,395 (1996): The use of a cone shaped (conical) bubbler is described in this patent.        i) Japanese Published Patent Application No. 2003/303772): This bubbler is a solid organometallic compound packing container having a flow direction switching pipe crossed through a partition plate that vertically divides the container.        
Unfortunately, none of the previously described bubbler designs have solved all the problems of solid organometallic delivery. None of the aforementioned bubbler designs are capable of providing a uniform delivery rate with a maximum pick-up of precursor material until substantially complete depletion of the vapor source occurs over a wide operational range. Each bubbler design has a limited range of parameters where it operates most efficiently. Breakthrough of non-saturated carrier gas occurs prematurely or gradually as the solid substrate is depleted. Premature breakthrough results in poor delivery efficiency and wasting of valuable organometallic product due to early removal and replacement with a new bubbler. A slow drop-off in percent carrier gas saturation can lead to production of sub-standard deposition layers if undetected during the deposition process.
The present invention solves the aforesaid problems.